The diamondback moth: a problem pest of brassica crops in Kenya

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1 The diamondback moth: a problem pest of brassica crops in Kenya G. N. Kibata Kenya Agricultural Research Institute, P O Box, Nairobi, Kenya Abstract Cruciferous crops are important vegetables in Kenya for local consumption, export and processing (dehydration and oil extraction). They grow well on a wide variety of soils wherever water is available. One major limiting factor to successful production of these vegetables is the diamondback moth (DBM), Plutella xylostella L. (Lepidoptera: Yponomeutidae). The larvae are voracious defoliators with a potential to destroy entire crops if not effectively controlled. Host resistance, endemic biocontrol agents and cultural practices have not been fully exploited in the management of DBM in Kenya. The easier option has been the use of spurious amounts of chemical pesticides. In the early s, organochlorines such as DDT afforded satisfactory control of DBM. In subsequent years, several carbamate-based products achieved excellent control of the pest. With the advent of synthetic pyrethroids in s, DBM appeared to have been completely subdued. Recent resurgences of the pest and progressive control failure herald intrinsic changes in DBM which make the pest less susceptible to the available chemical arsenal. This paper reports on pesticide screening trials, undertaken, on farm, to assess efficacy of various products on DBM. It also alludes to circumstantial evidence of pest resistance to popularly used pesticides and is a prognosis for development of more sustainable control strategies. Key words: DBM, chemical control, Brassica, Kenya. Introduction Brassica crops, which include cabbages, kale, brussel sprouts, cauliflower, broccoli and rape seed, are important vegetables in Kenya, largely grown for local consumption, export and processing (dehydration and oil extraction). While most of the production is in the small holder sector, commercial production is on the increase, especially, for cabbage. Available statistics indicate that, collectively, brassicas are grown on over 4, hectares with a production of 4, metric tonnes, valued at Kenya 4 million. (MALDM, ). The crops grow well on a wide range of soils and agro-ecological zones, wherever water is available. A major production constraint is the diamondback moth (DBM) Plutella xylostella L. (Lepidoptera: Yponomeutidae). The larvae of DBM are voracious defoliators which could destroy an entire crop if left uncontrolled. DBM was first recorded as a cosmopolitan agricultural pest in 4 (Harcourt, ). The presence of feeding stimulants (mustard glucosides) and the absence of chemical inhibitors restrict the oligophagous larvae of DBM to the mustard type of plants, the Cruciferae (Thornsteinson, ). It has been recognised that host plant resistance, use of natural enemies and management of the crop agroecology could alleviate problem of DBM (Ullyett, 4). Several natural enemies of DBM have been recorded in Kenya (Le Pelley, ; Kibata, ). The following organisms have been recorded as natural enemies of DBM in Kenya: Predators Birds Spiders Ants - Pheidole sp. Hover flies Syritta sp. and Melanostoma sp. (Diptera: Syrphidae) Parasitoids Diadegma molliplum (Holmgren) (= Stellenboschensis Cameron) (Hymenoptera: Ichneumonidae) Diadegma sp. (Hymenoptera: Ichneumonidae) Itoplectis melanospilla Cameron (Hymenoptera: Ichneumonidae) Hemiteles sp. (Hymenopteran: Ichneumonidae) Diplazon sp. (Hymenoptera: Ichneumonidae) Brachymeria apantelesi Ribsec (Hymenoptera: Chalcididae) Pteromalus sp. (Hymenoptera: Pteromalidae) Tetrastichus sokolowskii Kurdjumov (Hymenoptera: Eulophidae) Macrogaster sp. (Hymenoptera: Braconidae) Carducia plutella Emd (Diptera: Tachinidae) Entomophagous fungi Possibly Entomophthora sphaerosperma Fres. (= Zoophthora radicans, Brefeld) has been found to cause epizootics on DBM under favourable conditions. The fungus has potential to control DBM (Wang endo, Status of DBM and other pests of crucifers and their biocontrol 4

2 4). However, given the easier option of using chemical pesticides, little attempt has been made to exploit other pest control strategies. On a global scale the cost of managing DBM is currently estimated at USD billion through the use of pesticides (Talekar et al., ). In South East Asia, the prodigious use of pesticides to control DBM has resulted in the typical pesticide treadmill where shifting and changing of chemicals no longer solves the problem. In Kenya chemical control of DBM was successfully undertaken with organochlorine insecticides, such as DDT, lindane and endosulfan, in s. In later years carbamates became more popular, such as carbaryl, cartap hydrochloride, acephate and carbofuran. With the advent of synthetic pyrethroids in s, DBM became a problem of the past, as dramatic control of the pest was achieved (Kibata, ). Recently, however, the scencario has been gradually changing as more intensive production of brassicas and rampant usage of pyrethroids continues. Reports on control failure for DBM were on the increase, prompting further search for products with varied modes of action. This paper reports on recent trials undertaken to evaluate new and old products for this purpose. There is, however, no claim that the DBM problem is resolved. Materials and Methods Four series of trials were conducted at two farms in the north coast of Naivasha ( 4' 4"S, 4', "N, m asl) from April 4 to May. The trials design was CRBD with three replicates, whilst treatments ranged from to entries, inclusive of untreated checks (controls). Seedlings of cabbage (Brassica oleracea var. capitata) variety Gloria were transplanted into plots at row and plant spacing of cm and cm respectively. Inter block and inter plot paths were m and m, respectively. Plant stand counts for the four trials were,, and per plot, respectively, exempting guard rows. Fertilizer was applied as recommended for cabbage at transplanting and subsequently as top dressings. Once the seedlings were established, weekly samplings for immature DBM were undertaken on five cabbages per treatment plot. Application of test insecticides were made fortnightly using a CP knapsack sprayer with a hollow cone nozzle calibrated to deliver 4 litres of spray per hectare. At maturity all marketable cabbage heads were harvested, counted and weighed for assessment of yield. The pooled means of DBM counts for immatures over the crop season, numbers of marketable cabbage heads and yield was subjected to ANOVA and DMRT to assess effects of treatments on these parameters (Tables,,, 4). Results The collective results of the four trials demonstrated that there are significant differences between treatments (p <.) with regard to DBM infestation, numbers of marketable cabbage heads and related yield (Tables,,, & 4). Whilst the DBM infestation varied between the trials, higher infestation was observed in first and fourth trial runs. The trend was, however, consistent with respect to efficacious products in comparison with untreated control plots. Mean DBM infestation was highest in control plots where yield of marketable cabbage was also lowest in all instances. Effective products more than doubled the yield of cabbage, which confirms the need to evolve effective management strategies for DBM. On the basis of DBM suppression and cabbage yield the test products could be classified as follows: Very effective Secure (AC ) Fipronil - Insecticide, acaricide trifluoromethyl pyrrole carbonitrile - phenyl pyrazole insecticide, acaricide Prothiofos (Tokuthion ) - organophosphate (OP) BtXentari - Bacillus thuringiensis, Berliner subsp. aizawai Novaluron (MCW-) - Benzoylphenyl urea (IGR) Effective Methidathion - organophosphate Methomyl + Methidathion - carbamate/op Methomyl - carbamate Least effective Methamidophos - organophosphate Cypermethrin - pyrethroid Acephate - carbamate Fenpropathrin - pyrethroid Neem powder - Botanical Etofenprox (Trebon ) - non-ester pyrethroid Discussion Results of these trials subscribe to the view that pyrethroids which have been extensively used are no longer as effective as they used to be whilst products from new chemistry appeared extremely effective against DBM. Among the most effective products, Secure (AC ) is an insecticide and acaricide from Cyanamid based on 4-bromo--(4-chloropheny)--ethoxymethyl- -trifluoromethylpyrrole-=carbonitrile. Fipronil is a phenyl pyrazole insecticide acaricide from Rhone- Poulenc. Prothiofos is an old OP insecticide from Bayer while Bt Xentari is a new insecticide based on Bacillus thuringiensis, Berliner subspecies aizawai from the same company. Novaluron (MCW-) is a novel benzoylphenyl urea from Makhteshim Agan. Evidently, the modes of action of these products are 4 Proceedings: The Management of Diamondback Moth and Other Crucifer Pests

3 Table. Effect of treatments on DBM infestation and cabbage yield at Aberdare farm, (Trial period /4/4 to //4) (plant population ) Rate Mean Mean Mean g.a.i. ha - DBM marketable cabbage yield counts heads (kg) per plot. Untreated. a. a a. Acephate WP. fg def fg. Fipronil SC.. gh fg 4 hij 4. Fipronil SC 4. h 4 g 4 ij. Fipronil SC. i h k. Etofenprox Ec. bc ab ab. Chlorpyrifos 4 Ec 4.4 b bc 4 bc. Dichlorvos Ec 4. cd bc bc. Thiodicarb Ec.. e bcd bcd. Methomyl SP.4 f ef fg. Secure SC 44. i 4 g k. Cypermethrin Ec. e cdef efg. Methidathion 4 Ec 4. gh cdef ef 4. Methidathion + Methomyl Ec. de def fgh. Methidathion + Methomyl Ec. h def 4 ghi. Pyrethrins Ec. b cdef 4 efg. Bt Xentari g 4. h f ij. Methamidophos SC. f 4 bcde de. Methidathion + Methomyl Ec. f bcd cd Means followed by the same letter within the same column are not significantly different (P >.) by DMRT Secure proposed common name for AC Bt Xentari x I.U.mg - (Bacillus thuringiensis subsp. aizawai) g of product applied per hectare Table. Effect of treatments on DBM infestation and cabbage yield at Aberdare farm, (Trial period //4 to //4 (plant population ) Rate Mean Mean Mean g.a.i/ha DBM marketable cabbage Yield counts heads (kg) per plot. Untreated. a. a 44 a. Acephate WP. ef ab 4 cd. Fipronil SC..4 hi bc cde 4. Fipronil SC.. hij def 4 efg. Fipronil SC.. hij def efg. Etofenprox Ec. b a ab. Prothiofos Ec. hij def 4 gh. Azadirachtin. L. c a ab. Thiodicarb Ec.. b a ab. Methomyl SP. d ab bc. Secure SC 44. i def 44 h. Cypermethrin Ec. d ab bc. Methidathion 4 Ec. g bcd 4 cd 4. Methidathion + Methomyl Ec.44 h bcde de. -ditto-.4 hi bcdef de. -ditto-.4 ij cdef ef. Azadirachtin Ec 4 g/plant. fg abc 4 cd. Bt Xentari. ij def 4 gh. Methamidophos Sc. de ab bc Means followed by the same letter within the same column are not significantly different (P >.) by DMRT. Neem seed liquid extract (JAWAN ) from India applied at. litres per hectare. Secure proposed common name for AC. 4 Neem seed powder (locally produced) % a.i. applied at g/plant. Bt (Xentari) x I.U. mg - (B.t. subsp. aizawai) applied g/ha. different from that of pyrethroids which are relatively ineffective on DBM at Naivasha. From results of the last trial it was shown that DBM infestation was significantly related (P <.) with numbers of marketable cabbage heads (r =.) (Y =..x) as well as the marketable weight of cabbage (r =.) (Y =..x). This is an expected outcome as heavily infested cabbage failed to form heads or formed heads which could not be marketed. It is therefore pertinent to conclude that DBM causes substantial economic loss in cabbage Status of DBM and other pests of crucifers and their biocontrol 4

4 Table. Effect of treatments on DBM infestation and cabbage yield at Boffar farm, Naivasha, (Trial period //4 to 4//4) (plant population ) Rate Mean Mean Mean g.a.i./ha - DBM marketable cabbage counts heads yield (kg) per plot. Untreated. a 4 a abc. Acephate WP. d abc cd. Fipronil SC.. fg abc 4 gh 4. Fipronil SC..4 hi e gh. Fipronil SC.. j cd gh. Etofenprox Ec. b ab a. Prothiofos Ec. j e j. Azadirachtin. L. c ab bc. Thiodicarb Ec.. c ab bc. Methomyl SP. d abc cd. Secure SC 44. ij e i. Cypermethrin Ec. de bc fg. Methidathion 4 Ec. fg abc fg 4. Methidathion + Methomyl Ec. gh abc c. Novaluron Ec. ef abc def. Novaluron Ec. gh abc def. Azadirachtin 4 g/plant. de abc fgh. Bt Xentari g. gh de hi. Methamidophos SC. fgh abc cde. Bt Xentari g. d abc efg Means followed by the same letter, in the columns, are not significantly different (P >.) by DMRT. Neem seed liquid extract (JAWAN ) from India applied. litres ha - Secure proposed common name for AC. 4 Neem seed powder (locally produced) applied at g/plant. & Bt (Xentari) x I.U. mg - B.t. subsp. aizawai applied at g and g product ha -. Table 4. Effect of treatments on DBM infestation and cabbage yield at Aberdare farm, (Trial period /4/ to //) (plant population ) Rate Mean Mean Mean g.a.i./ha - DBM marketable cabbage yield counts heads (kg) per plot. Untreated. a. h. g. Acephate. ef. h.4 g. Fipronil SC.. g 4. de. def 4. Fipronil SC.. ij. c 4. cd. Fipronil Sc.. k. ab. ab. Prothiofos Ec. de. h. g. Secure SC 44. l 4. a. a. Cypermethrin Ec.4 b. h. g. Methidathion 4 Ec. hi. efg. defg. Methidathion + Methomyl Ec. gh. fg. efg. Novaluron Ec. h 4. fg. def. Novaluron Ec 4. i. d 4. cde. Bt Xentari 4 g. hij 4. ef. def 4. Bt Xentari g. k. b. bc. Methamidophos Sc.4 g.g 4. fg. Triflumuron 4 SC 4. dc. h. g. Fenpropathrin Ec. e. h.4 g. Fenpropathrin Ec. cd. h. g. Beta-cyfluthrin Ec. f. h.4 g. Endosulfan 4 FL 4. c. h. g. Carbosulfan Ec. de. h. g Means followed by the same letter within columns, are not significantly different (P >.) by DMRT. Acephate (% pellets) Secure proposed common name for AC 4 Bt (Xentari) x I.U. mg - Bt subs. aizawai applied at and g ha - Proceedings: The Management of Diamondback Moth and Other Crucifer Pests

5 Mean yield in kg Figure. Mean counts of DBM and yield in various treatments (refer table ) in Trial production. Some commercially available pesticides are no longer affording the level of desired control on DBM. Products under development, whose mode of action is novel, appear to be extremely effective on DBM. This heralds a warning that pest resistance to popular insecticides may have developed in the diamondback moth. The need to develop a comprehensive strategy for better management of DBM is therefore advocated if successful production of Brassica crops is to be sustained in Kenya. It is also noted that previous work on identification of other major mortality factors for DBM identified several natural enemies. These were only collected from farms where no pesticides were being used or where there were adequate refugia in wild cruciferous weeds or neglected Brassica crops. The natural enemies did not however appear to confer satisfactory suppression of DBM on their own. An integrated approach to the management of DBM should therefore be explored. Acknowledgement The author wishes to gratefully acknowledge the assistance of Messrs S. O. Pete and J. W. Kamau who assisted in the implementation of the trials and Margaret Gitonga who diligently typed the manuscript. The work would have been impossible without the financial and moral support of the KARI/ODA Crop Protection Project. Preparation of this paper has been duly authorised by the Director, Kenya Agricultural Research Institute (KARI). The author is gratefully indebted to the GTZ-IPM Horticulture project for the sponsorship to this conference. References Harcourt, D.G. (). Biology of cabbage caterpillars in Eastern Ontario. Proc. Entomol. Soc. Ont. :. Kibata, G.N. (). Control of Diamondback moth caterpillars on cabbage p.. Senior Entomologist s Annual Report. pp. Le Pelley, R.H.. Index of insect hosts of parasites: Plutella maculipennis - Plutellidae p.4. In Agricultural Insects of East Africa. E.A. High Comm. Nairobi, Kenya pp. Status of DBM and other pests of crucifers and their biocontrol

6 Mean yield in kg Figure. Mean counts of DBM and yield in various treatments (refer table ) in Trial Mean yield in kg Figure. Mean counts of DBM and yield in various treatments (refer table ) in Trial 4 Proceedings: The Management of Diamondback Moth and Other Crucifer Pests

7 Mean yield in kg Treatment 4 4 Figure 4. Mean counts of DBM and yield in various treatments (refer table 4) in Trial 4 MALDM (). Crop Production statistics. Min. of Agric. Liv. Dev & M. Kenya. Talekar, N.S., J.C. Yang, and S.T. Lee, (). Introduction of Diadegma semiclausum to control diamondback moth in Taiwan. In Diamondback moth and other crucifer pests, Proceedings of the Second International Workshop, Tainan, Taiwan. Talekar, N.S. (ed.), pp.. AVRDC. Thorsteinson, A.J. (). The chemotactic reponses that determine host specificity in an oligophagus insect (Plutella maculipennis (Curt) Lepidoptera). Can. J. Zool. :. Ullyett, G.C. (4). Mortality factors in populations of Plutella maculipennis Curtis and their relation to the problem of control. Entomol. Mem. Dept. Agr. S. Africa. :. Wang endo, L. (4). Interactions between the insect growth regulator, Flufenoxuron (CASCADE R; ); the entomopathogen, Zoophthora radicans Brefeld and the Diamondback moth Plutella xylostella L. MSC thesis, Univ. of Bristol, United Kingdom. Status of DBM and other pests of crucifers and their biocontrol